The lean operation and relatively cool operating temperatures of diesel engines create an environment where incomplete combustion produces particle emissions and soot build up that necessitates exhaust-gas aftertreatment including particle filters. This can also be true of direct injected spark ignition engines. This is especially the case with turbocharged engines that have higher NOx emissions. Exhaust-gas recirculation combats the higher NOx emissions but at the expense of soot production. More efficient particle filtration could provide both minimized particle emissions and maximized energy efficiency by reducing backpressure through the filtration device.
DE 40 04 424 A1 discloses a device for purification of the exhaust gases of diesel engines, in which device, the exhaust line of the engine is divided into two branch lines which can be alternately shut off by means of a switching device and one of which leads through a soot filter. Downstream of the branch line containing the soot filter, the other branch line, which serves as a bypass line, merges again to form an end line. The branch line that leads through the soot filter is opened above a predetermined partial engine load value, as well as at full engine load, by means of the switching device, which can be actuated as a function of the engine load. To be able to ensure practically complete purification of the exhaust gases, an oxidation catalytic converter is installed downstream of the switching device in the bypass line or in the end line. If the oxidation catalytic converter is installed in the end line, all of the exhaust gas flows through it.
DE 10 2004 049 511 A1 is concerned with a semi-active heat-exchanging silencer, downstream of which is positioned a catalytic converter. In order to prevent heat losses from the exhaust gases after starting of the internal combustion engine and to effect heat losses when the engine is at operating temperature, via a compact component, two different flow paths are provided in the silencer. One of said flow paths is of heat-insulated design whereas the other flow path is designed such that the exhaust gases are cooled.
DE 10 2005 019 466 A1 in turn discloses a diesel particle filter for an exhaust system of an internal combustion engine, said diesel particle filter having at least one housing and at least one filter body. The filter device comprises at least two filter bodies arranged fluidically in series. The filter bodies are arranged spaced apart from one another in a common housing. Between the filter bodies there are provided chambers into each of which externally connected lines open out.
Exhaust-gas aftertreatment devices in the form of particle filters or soot filters require periodic regeneration through burn-off of the captured soot particles. To be able to perform the regeneration, the exhaust-gas temperature is raised to a value above, for example, 550° C. in order to initiate the combustion of the captured soot particles. In the case of lean-burn internal combustion engines, for example, diesel engines, said temperature levels are attained during normal operation of the internal combustion engine under full load if the soot filter is in a close-coupled arrangement. Close-coupled means that the soot or particle filter is arranged so close to an exhaust-gas outlet of the internal combustion engine that heat losses of the exhaust gases are minimized, but also that a desired passive regeneration is attained. Since the soot filter is arranged very close to the exhaust-gas outlet where there is little installation space available, design parameters, that is to say the size and capacity of the soot filter, are restricted. Furthermore, the configuration of the exhaust lines (cones, bends) may lead to a sub-optimal flow distribution. This causes a considerable pressure drop across the soot filter resulting in high throughflow rates, which has an adverse effect on the fuel consumption of the internal combustion engine, in particular during, so-called, highway driving.
To, at least partially, eliminate said problems, DE 10 2009 029 259 A1 proposes an exhaust-gas aftertreatment system of the type mentioned in the technical field. In said exhaust-gas aftertreatment system, within a first soot filter, there is arranged a passage line which can be shut off via a control element, which is closed in the circumferential direction, is free from filter elements, and which extends all the way through the particle filter element of the first soot filter. Here, at least one second soot filter is positioned downstream of the first soot filter. It is thus the case that two soot filter elements are arranged in series in the exhaust line or in an exhaust section, such that a low pressure drop can be attained despite a high throughflow rate. Therefore, improved fuel consumption can be attained in conjunction with an increased back pressure caused by the one or more soot filters.
A disadvantage of such a system is that, with the “downsizing” of the swept volume of engines which has been increasingly pursued recently, the exhaust-gas temperature is often no longer adequate, in particular in the lower load range, to generate the exhaust-gas temperature required for the regeneration of the particle filter. At the same time, relatively high exhaust-gas temperatures, and therefore greater quantities of emissions, in particular of nitrogen oxides, are encountered in such engines at medium load and full load. To reduce nitrogen oxide emissions, increased exhaust-gas recirculation is performed, which comes at the expense of soot emissions. However, if the required regeneration temperatures of the soot particle filter are not exceeded at corresponding time intervals, the soot filters reach their capacity limit relatively quickly, and can then accommodate no further soot.
An increase in the soot accommodation capacity of such exhaust-gas aftertreatment systems by increasing the size thereof is however possible and expedient within certain limits. Firstly, this is generally associated with an increased space requirement, wherein the space availability in the under-floor region of passenger motor vehicles in particular is limited. Furthermore, this would also increase the dynamic pressure in the exhaust tract, which is likewise, acceptable within certain limits. Consequently, the exhaust-gas aftertreatment systems hitherto known from the prior art are often not capable of reducing the emissions of such engines to a satisfactory extent over the different operating states, in particular in light of the intensely fluctuating exhaust-gas temperatures and the resulting fluctuating emission profile.
It is an object of the present disclosure to modify an exhaust-gas aftertreatment system such that satisfactory exhaust-gas treatment is attained, in particular with regard to soot emissions, even in the case of engines of reduced swept volume. In one example, said object is achieved via an exhaust-gas aftertreatment system for an internal combustion engine, having an exhaust line containing a filter arrangement which comprises a first particle filter element and a second particle filter element, wherein the first particle filter element is equipped with an active regeneration device for restoring its filtration performance.
In this way, it is possible to provide particle filtration and regeneration of the particle filter element without the space constraints of a close-coupled particle filter. Additionally, because of the affixed active regeneration device on the first particle filter element, soot particle overloading can be reduced and there is no need for increased soot load capacity. The use of the two particle filter elements of the present disclosure furthermore decrease backpressure compared to a system with a single particle filter element.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.